Power control is one of the most important requirements for cellular network systems, such as Universal Mobile Telecommunications Systems (UMTS) employing WCDMA (Wideband Code Division Multiple Access), in particular on the uplink, i.e. from a mobile station to a base station.
Without a suitable power control mechanism a single mobile station could block an entire cell, if that mobile station is overpowered.
Given that two mobile stations are operating within the same frequency, the base station can only separate both mobile stations by their respective spreading codes. If the first one of both mobile stations is operated near by the edge of the cell it may suffer a path loss, e.g. 60 dB above that of the second mobile station which is assumed to be nearby the corresponding base station.
If there would be no power control mechanism for the mobile stations, controlling the respective power levels of both mobile stations, such that the base station receives signals from both mobile stations at about the same level, the mobile station nearby the base station could easily “overshout” the mobile station at the edge of the cell. Thus, the mobile station nearby the base station could block at least a large part of the cell.
Therefore, it is desirable to equalize the received power per bit of all mobile stations (received by the base station) at any time in order to maximize network capacity.
A known solution for this kind of problem is the so-called fast closed-loop power control wherein in the uplink the base station carries out estimates of the received signal-to-interference ratio (SIR) and compares the estimated SIR to a target SIR. If the SIR measured for a certain mobile station is higher than the target SIR, the base station will send a power control command to the respective mobile station indicating the mobile station to lower the power. If, however, the measured SIR is too low, the base station will send a power control command to this mobile station indicating the mobile station to increase the power.
This mechanism of measuring SIR, sending power control commands, and adjusting the transmission power by the mobile station is performed at a rate of 1500 times per second, i.e. with a frequency of 1.5 kHz for each mobile station. Thus, this mechanism operates faster than any significant change of path loss could possibly happen. However, this mechanism applies only to mobile station velocities lower than approximately 50 km/h. For velocities higher than this, other features like outer loop power control should take over and adjust the EB/N0 target to ensure proper operation. Therefore, this power control mechanism is called fast transmit power control (TPC).
As a result, the base station informs the mobile station by corresponding power control commands which power level is to be used for transmission in the next slot (660 μs corresponding to 2560 chips). These power control commands indicate to the mobile station to increase or decrease the transmission power by a fixed step size, e.g. 1 dB.
Furthermore for enhancing the uplink performance, it has been suggested in 3GPP (3rd Generation Partnership Project) to use hybrid automatic repeat control (H-ARQ), wherein the base station in case of a faulty reception of a data block can request a retransmission of the specific data block.
When requesting the retransmitted data block it is possible to combine the retransmitted version of the faulty data block with the first version of the faulty data block. Thus, the probability of detecting the data block correctly is increased. In order to achieve a high probability for correct detection of this data block, the retransmitted version of the data block is transmitted at the same power level as the first version of the respective faulty data block. Thus, there is a chance that such a retransmitted data block is finally received with a quality that is higher than the required quality. However, this is a waste of cell capacity.